Power control device, apparatus and method of controlling the power supplied to a discharge lamp

ABSTRACT

The present invention relates to a power control device for controlling the output power supplied to a discharge lamp operated by an electrical power supply, comprising power level determining means for determining the actual lamp power level, error determining means for determining the error between the determined lamp power level and a specified reference power level, and output power determining means for maintaining the output power level supplied by the electrical power supply to the lamp if the error falls within a specified window and for adjusting the output power level supplied by the electrical power supply to the lamp towards said reference power level if the error falls outside the specified window.

This Application is a National Phase Application under 35 U.S.C. 371claiming the benefit of PCT/IB02/01845 filed on May 24, 2002, which haspriority based on European Applications (EPO) No. 01202067.3 filed onMay 31, 2001 and No. 01204607.4 filed on Nov. 29, 2001.

The present invention relates to a device and a method of controllingthe power supplied to a discharge lamp, such as fluorescent lamps,halogen lamps etc. operated by an electrical power supply.

Power control devices or ballasts are widely used for controlling thepower supplied to the discharge lamp. Ballasts can be employed tooptimize the preheating and ignition of the discharge lamp, to maintaina constant power to the electric discharge lamp for the purpose ofmaintaining a selected light intensity or for the purpose of controlleddimming to a fixed, but adjustable, power level of the discharge lamp.

U.S. Pat. No. 5,910,713 discloses an analog power control system whereina lamp current detecting circuit provides a signal representative of thecurrent in the lamp, which signal is used in a feedback loop to adjustthe power supply to the lamp. This power control system aims tostabilize the current in the lamp. However, adjustment of the powersupply to stabilize the lamp power is not realized.

U.S. Pat. No. 4,928,038 discloses an analog power control circuit with apower supply controlled by the switching frequency of a power switch.The power supplied to the lamp is controlled on basis of the detectedcurrent flowing through the power switch itself instead of the currentin the lamp.

U.S. Pat. No. 5,806,055 discloses a digital ballast (power controldevice) wherein analog control loops are approximated by digital controlloops. The digital ballasts provide a relatively low cost power control.Digital ballasts are versatile as compared to the analog ballasts andallow for easier implementation of complicated control and timingprocesses.

Generally the power source of the lamp is the mains and consequently thesignal provided by the source contains a ripple (generally 100 Hz or 120Hz). This ripple will also be present on the control loop signal, suchas the measured lamp voltage and/or the measured lamp current. Thedigital control using the control loop signal will try to cancel theripple. This can cause mixing of the sampling frequency and the ripplewhich may cause instability of the control loop resulting in visiblelight flicker.

The object of the present invention is to provide a power control deviceand a method for controlling the power supplied to a discharge lamp withimproved stability.

According to a first aspect of the present invention a power controldevice for controlling the output power supplied to a discharge lampoperated by an electrical power supply is provided, comprising:

power level determining means for determining the actual lamp powerlevel;

error determining means for determining the error between the determinedlamp power level and a specified reference power level;

output power determining means for maintaining the output power levelsupplied by the electrical power supply to the lamp if the error iswithin a specified window and for adjusting the output power levelsupplied by the electrical power supply to the lamp towards saidreference power level if the error is outside the specified window. Theoutput power level is adjusted only if the difference between thereference power level, for example the (dimming-)level set by the userof the lamp, and the actual power level exceeds the specified value.This value is chosen so as to be larger than the ripple on the powerconsumed by the lamp. If the difference between the reference powerlevel and the measured lamp power level is small, this difference issupposed to be caused by the ripple and consequently no correctiveaction is taken.

In a digital power control device the actual power level and theresulting error are determined repeatedly, for example with a clock rateof 500 Hz, and the output power level is adjusted iteratively towardsthe reference level.

On the one hand the window should be wide enough to get rid of theripple. On the other hand the window should be narrow enough to providea sufficient power control of a dimmed lamp. In a preferred embodimentthe width of the window is therefore determined to be dependent on thespecified reference power level. As the ripple on the DC supply voltagedecreases with decreasing output power level because the currentconsumption of the power supply drops at low output power, the window istightened towards lower reference power levels.

In a further preferred embodiment the output power determining meanscomprise means for varying the window width between a maximum windowwidth and a minimum window width, the ratio of which is preferablyapproximately 1/10 or more. A minimum window width should be maintainedto cancel limit cycle oscillations which would occur due to lack ofinput and/or output resolution (for example determined by the resolutionof the A/D- and D/A-converters). Therefore the maximum and minimumwindow widths are variable, dependent on the resolution of theelectronic circuitry (micro controller) used. In case of amicrocontroller with high resolution, a large ratio is preferred.

In a further preferred embodiment the output power determining meanscomprise means for determining the reference power level on basis of aprestored nominal lamp power level and a dimming level, which is inputto the output power determining means. This makes the power controldevices suited for different types of lamps (50 W, 60 W, etc). Changingbetween lamp types takes the substitution of the value of the nominalpower only, which is preferably stored in a microcontroller to bedescribed hereafter, to adapt the power control device to the specifictype of lamp.

In further preferred embodiment one or more of the corrections aredependent on the error level. When the error is large, the controldevice will iteratively correct the output power using a relativelylarge step size, while when the error is small the control deviceiteratively corrects the output power by using a relatively small stepsize.

In a further preferred embodiment the output power determining means anderror determining means comprise a programmable microcontroller (MC)connected to an interface circuit (IFC). The microcontroller isprogrammable by storing software in its memory. Adaptation of thecontrol device to different lamp types and implementation of complicatedcontrol and timing processes can be achieved by adaptation of thesoftware running on the microcontroller.

In a preferred embodiment the output power determining means can beconnected to one or more switching elements of the electrical powersupply for controlling the output power by controlling the switching ofthe switching elements. The output power supplied to the lamp is in thisembodiment dependent on the cycle of the switching elements.

According to another aspect of the present invention an apparatus isprovided for supplying power to a discharge lamp, preferably comprisingthe earlier described power control device, the apparatus comprising:

an electrical duty cycle controlled power supply for supplying power tothe lamp;

power level determining means for determining the actual level of thelamp power;

error determining means for determining the error between the determinedlamp power level and a specified reference power level;

output power determining means, connected to the power supply forcontrolling the duty cycle of the power supply so as to adjust theoutput power to be supplied to the lamp towards said reference powerlevel only if the error falls outside a specified window. In thisapparatus preferably the earlier mentioned power control device isapplied. In a preferred embodiment the DC power supply is controllableand the power determining means control the output voltage (UDC) of theDC power supply as to adjust the output power. In this embodiment asupply voltage variation method is applied for controlling the outputpower. In yet another preferred embodiment the operation frequency ofthe power supply is controllable and the power determining means controlthe output voltage of the DC power supply so as to adjust the outputpower. In this embodiment a frequency variation method is applied forcontrolling the output power.

According to another aspect of the present invention a method isprovided of controlling the power supplied to a discharge lamp operatedby an electrical power supply, comprising:

determining the actual power level of the power consumed by the lamp;

determining the error between the actual lamp power level and aspecified reference power level;

if the error falls within a specified window, maintaining the outputpower level supplied to the lamp;

if the error falls outside the specified window, adjusting the outputpower level supplied to the lamp towards said reference power level, thewidth of the window being preferably dependent on the specifiedreference power level.

Further advantages, features and details are given in the followingdescription of a preferred embodiment of the invention. In thedescription reference is made to the annexed figures.

FIG. 1 is a block diagram showing the preferred embodiment of thepresent invention for operating the discharge lamp;

FIG. 2 shows an integrating window to be applied on the deviationbetween the output power and reference power;

FIG. 3 shows two integrating windows to be applied on the deviationbetween the output power and reference power;

FIG. 4 shows a graph of the ripple on the lamp power when the lamp isoperated at a nominal power level and a dimmed power level;

FIG. 5 shows the window width as function of the dimming level for agliding window.

The lamp power supply according to the preferred embodiment of theinvention is a duty cycle controlled power supply of the constantfrequency pulse width modulation (PWM) type, which uses the samefrequency for ignition, normal operation and dimmed operation of thelamp. In the embodiment shown in FIG. 1, the power supply is ahalf-bridge, which produces a square wave signal and serves for ignitionand normal/dimmed operation of the lamp.

The power supply operates in the symmetrical mode. The duty cycle of thetwo switching elements are equal, their on-times being separated fromeach other by ½ of the switching period. In the ignition phase the L-Ccombination L_(lamp), C_(lamp) is unloaded which generates a highvoltage across the lamp. This causes ignition of the lamp. In the burnphase the L-C combination L_(lamp) and C_(lamp) is loaded by the lamp.The power delivered to the lamp is determined by duty cycle. Hence, thelamp power supply is controlled by one parameter, the duty cycle for theswitching elements.

In the block diagram of FIG. 1 it is shown that a diode bridge B1 isconnected to the mains (220 V AC). The bridge B1 rectifies the mains andprovides a DC supply voltage of about 300 V.

For driving the lamp a half-bridge drive circuit is shown, wherein theswitching elements are formed by two power transistors (power FETs) Q1and Q2. The gates of the switching elements Q1 and Q2 are driven bydriver signals GHB1 and GHB2 originating from a control circuit to bedescribed hereafter.

Further are shown a DC blocking capacitor CDC, a LC-combinationL_(lamp), C_(lamp) for driving the lamp, and a microcontroller MCconnected to an interface circuit (IFC) for providing the controlsignals GHB1 and GHB2 for power transistor Q1 and Q2 respectively. Asthe microcontroller operates on a relatively low voltage (typical 5 Vsupply voltage), the input signals must be in the range from 0 to 5V andconsequently the output signals that the microcontroller can deliver arealso in this range. Consequently, the interface circuit (IFC) isprovided for converting voltages and currents into usable indicationsignals for the microcontroller (MC) and for converting control signalsfrom the microcontroller (MC) into usable driver signals for theswitching elements Q1 and Q2. The microcontroller MC is provided withA/D-converters and D/A converters, read-only memory (ROM), programmableor non-programmable, and/or random access memory (RAM). In the memorycontrol software is stored.

Although not shown in FIG. 1, electrode heating circuits, which are usedto preheat the electrodes before ignition of the lamp, and various typesof protection circuits, etc. can also be provided.

The microcontroller MC outputs, under software control, a square wave,which is averaged in the interface circuit with an RC-filter to rule outthe ripple component. The resulting DC-voltage is used by the interfacecircuit (IFC) to generate the driver signals GHB1 and GHB2 for theswitching elements Q1 and Q2 respectively. Consequently, the duty cycle,with which the power supply to the lamp is controlled, is determined bysoftware stored in the memory of the microcontroller.

The functions of stabilization of the power or current in the lamp, theoptimization of the ignition, preheating and electrode heating, theadaptation to different lamp types, can be achieved by adapting thesoftware running on the microcontroller. These functions are implementedby a digital control loop for which the microcontroller performsmeasurements of a plurality of physical quantities such as the currentin the lamp, the voltage across the lamp, the supply current and supplyvoltage.

I_(lamp) is the current running in the lamp. I_(lamp) can be determinedin various ways. In the embodiment of FIG. 1, I_(lamp) is determined bya lamp current transformer T, the primary windings of which areconnected between an electrode of the lamp and ground. The voltage ofthe secondary windings of the lamp current transformer T is rectified ina bridge circuit (not shown) and averaged. The resulting signal isrepresentative of the lamp current I_(lamp).

U_(lamp) is the actual voltage across the lamp. U_(lamp) can bedetermined in various ways. In the embodiment of FIG. 1, U_(lamp) isrepresented by the voltage taken from a high-ohmic divider and rectifiercircuit (DRV).

I_(supply) is represented by the averaged voltage across the shuntresistor of divider D_(I), while U_(supply) is represented by theaveraged voltage from divider D_(U).

The signals I_(lamp), U_(lamp), U_(supply) and I_(supply) are fed to theinterface circuit (IFC) that converts the signals into usable indicationsignals for the microcontroller.

The actual lamp power can be calculated by simultaneously measuringvoltage U_(lamp) across the lamp, measuring the current I_(lamp) runningin the lamp and subsequently, multiplying of the measured voltageU_(lamp) and current I_(lamp). This multiplication is performed in themicrocontroller. It also conceivable to calculate an averaged powerlevel by applying for example the following exponential digital filter:P _(lamp,n) :=P _(lamp,n-1)*(1−a)+a*I _(lamp,n-1) *U _(lamp,n-1)wherein P_(lamp,n) is the power of the lamp value calculated for time n,P_(lamp,n-1), U_(lamp,n-1) and I_(lamp,n-1) are the power, the voltageand the current for time n-1, and a is a constant (0<a<1).

The thus obtained control input power P_(lamp,n) is compared to areference power level P_(ref), which represents the actual desired powerlevel (target level). The reference power level is obtained bymultiplication of the nominal lamp power, which is prestored in thememory of microcontroller MC and is dependent on the specific lamp used,and one of a number of prestored values representative of the dimminglevel of the lamp. The dimming level can be set in a variety of ways,for example by adjustment of a switch (not shown) to be operated by theoperator.

The lamp power control procedure implemented by the software running onthe microcontroller is aimed to maintain the lamp power at the valueaccording to the reference power level or dimming level. The controlprocedure can be realized by applying fuzzy rules sets, morespecifically by applying the fuzzy rules in an integrating windowprocess.

In the integrating window process the magnitude and sign of thedeviation (error) of the measured power level from the reference powerlevel determines which action is to be taken. In FIG. 2 a window isshown running from −W/2 to +W/2. If the deviation is inside the {−W/2,+W/2} window, no corrective action is taken. If the deviation is outsidethe {−W/2, +W/2} window, the microcontroller takes a corrective action,resulting in a corrected value of the output of the microcontroller.This results in corrected values of the duty cycles of GHB1 and GHB2 andconsequently the output lamp power P_(lamp).

Above a description is given of how the microcontroller implements anintegrating window control process using only one integrating window. Ina further preferred embodiment the microcontroller implements anintegrating window control process using two or more windows, as isshown in FIG. 3. If, for example, the deviation is inside a first{−W₁/2, +W₁/2} (sub) window, no correction is applied. If the deviationis outside the first (sub) window {−W₁/2, +W₁/2}, but inside a secondwindow {−W₂/2, +W₂/2}, a first correction C₁ is applied, while if thedeviation is outside the {−W₂/2, +W₂/2} window, a second correction C₂,larger than the first correction C₁, is applied. In the embodiment shownthe corrections C₁ and C₂ are implemented by increasing or decreasingthe output power by respectively a relatively small and a relativelylarge step size. If for example the operator operates the abovementioned switch and sets the dimming level and hence the referencepower level to half of its original value, this causes a negativedeviation outside the relatively wide window as a result of which themicrocontroller responds with a fast decrease of the output power level.After a while the deviation will reach the range within the relativelywide window, but outside the relatively narrow window as a result ofwhich a slow decrease, or increase if the deviation becomes positive, ofthe output power level occurs.

In the above embodiment the corrections are implemented as relativelysmall and relatively large step sizes of constant value. This means thatthe correction is independent on the deviation (error) of the measuredpower level from the reference power level. However, in anotherembodiment the output power supplied to the lamp, or at the least theduty cycle of the power supplied to the lamp satisfies:P _(n) =P _(n-1) +K _(p)(E _(n) −E _(n-1))+K _(i) E _(n)

wherein P_(n) is (the duty cycle of) the output power level supplied tothe lamp on time n, P_(n-1) is (the duty cycle of) the output powerlevel supplied to the lamp of the current sample, E_(n) and E_(n-1) theerror of the current sample and of the previous sample, K_(p) is theproportional gain and K_(i) is the integrating gain. When the gainfactors are nullified for error signals satisfying −W/2<E<+W/2, then wehave a one window integrating/proportional digital control. This is alsoapplicable to two or more windows. When the gain factors are nullifiedfor error signals satisfying −W₁/2<E<+W₁/2, have a relatively smallvalue if −W₂/2<E<−W₁/2 or if +W₁/2<E<+W₂/2 and have a relatively largevalue if E<−W₂/2 or E>+W₂, then a two window integrating/proportionaldigital control is achieved. In this embodiment the correction of theoutput power is dependent on the error E and the process of iteration tocorrect the output power will converge in a relatively short time.

As the DC-source of the power supply is the rectified mains, the signalprovided by the source contains a ripple (generally 100 Hz or 120 Hz).This ripple will also be present on the measured lamp voltage U_(lamp)and measured lamp current I_(lamp) and consequently on the calculatedduty cycle of lamp power P_(lamp). The digital control will try tocancel the ripple. This can cause mixing of the sampling frequency andthe ripple which may cause instability of the control loop resulting invisible light flicker. Therefore the window must have sufficient widthto keep the control loop stable. To lose the ripple on the measured lamppower, the window should have a width of at least 10% of the nominalpower of the lamp (i.e. W=0.1*P_(nominal)). A high frequency powersupply for a lamp of 50 W nominal power needs for example an anti-ripplewindow of +/−2.5 W (i.e. W=2.5). If the output power level is dimmed to5 W and the same window would have been applied, the control tolerancewould be 2.5 W to 7.5 W. In the latter case the window is so wide thatpower control is insufficient.

The ripple on the DC supply voltage decreases with increasing dimmingbecause the current consumption of the high frequency power generatordrops at low output power. FIG. 4 shows the ripple on the DC supplyvoltage to the lamp, in case it is driven at its nominal power of 50 Wand in case it is driven at a dimmed power level of 5 W. The maximumripple at nominal power is approximately 5 W, which is about 10% of thenominal power. Hence, a window from −2.5 W to +2.5 W (W₁=5 W) issufficient to keep the control loop stable. The ripple at the dimmedpower level of 5 W is approximately 50 mW, which is about 0.1% of thenominal power of the lamp. In this case a window only ranging from −25mW to +25 mW (W₁=50 mW) would be sufficient to keep the control loopstable. Therefore the window is tightened towards a higher degree ofdimming.

A minimum window width should, however, be maintained to cancel limitcycle oscillations which would occur due to lack of input and/or outputresolution (for example determined by the resolution of the A/D- andD/A-converters).

In the preferred embodiment the window width is prestored in the memoryof the microcontroller (MC) as function of the reference power or asfunction of the dimming level.

In the memory tables containing a plurality of window width values andcorresponding dimming level values are stored, which are retrieved fromthe memory depending on the dimming level set by the operator. FIG. 5shows a continuous curve representing the window width as function ofthe dimming level of the lamp. When the lamp is operated at nominalpower of 50 W, a maximum window width W₁ of 5 W is applied. The controltolerance is +47.5 W to +52.5 W, enabling a sufficient power control.When the lamp is operated at a dimmed power level of 5 W, the windowwidth glides iteratively to a window width W₁ of 1 W, i.e. a decrease toapproximately ⅕ of it's maximum size. The control tolerance in this caseis +4.5 W to +5.5 W, which enables a sufficient power control

When the lamp is operated at a further dimmed power level of less than10% of the nominal power, the window width is further decreased untilthe width reaches the minimum window width which inter alia is dependenton the resolution of the microcontroller and its A//D- andD/A-converters.

FIG. 5 shows a window width that linearly decreases with decreasingoutput power. However, a non-linear decrease of the window width can beadvantageous, for example a relatively slow decrease in the region ofthe maximum output power and a relatively fast decrease in the region ofthe minimum output power.

1. Power control device for controlling an output power supplied to adischarge lamp operated by an electrical power supply, comprising: powerlevel determining means for determining an actual lamp power level;error determining means for determining an error between the determinedlamp power level and a specified reference power level; output powerdetermining means for maintaining an output power level supplied by theelectrical power supply to the lamp if the error falls within aspecified window and for adjusting the output power level supplied bythe electrical power supply to the lamp towards said reference powerlevel if the error falls outside the specified window, wherein saidreference power level is a determined by a preset nominal lamp power andan input dimming level.
 2. Power control device according to claim 1,wherein a width of the window exceeds a ripple on a lamp power.
 3. Powercontrol device according to claim 1, wherein a width of the window isdependent on the specified reference power level.
 4. Power controldevice according to claim 1, wherein the output power determining meanscomprise means for decreasing the window width towards low referencepower levels and increase a window width towards high reference powerlevels.
 5. Power control device according to claim 1, wherein the outputpower determining means comprise means for iteratively adjusting theoutput power level with a first correction if the error is outside thewindow, and maintaining the output power level if the error is insidethe window.
 6. Power control device according to claim 5, wherein one ormore of the corrections are dependent on the error level.
 7. Powercontrol device according to claim 1, the power level determining meanscomprising: means for determining an actual voltage across the lamp;means for determining an actual current in the lamp; means fordetermining the actual power level from the actual voltage and actualcurrent.
 8. Power control device according to claim 1, wherein theoutput power determining means and error determining means comprise aprogrammable microcontroller (MC) connected to an interface circuit(IFC).
 9. Power control device according to claim 1, wherein the outputpower determining means is connectable to one or more switching elementsof the electrical power supply for controlling the output power suppliedto the lamp by controlling the switching of the switching elements. 10.Power control device for controlling an output power supplied to adischarge lamp operated by an electrical power supply, comprising: powerlevel determining means for determining an actual lamp power level;error determining means for determining an error between the determinedlamp power level and a specified reference power level; and output powerdetermining means for maintaining an output power level supplied by theelectrical power supply to the lamp if the error falls within aspecified window and for adjusting the output power level supplied bythe electrical power supply to the lamp towards said reference powerlevel if the error falls outside the specified window; wherein theoutput power determining means comprise means for varying a window widthbetween a maximum window width and a minimum window width, a ratio ofwhich is preferably approximately 1/10 or more.
 11. Power control deviceaccording to claim 10, wherein the ratio of the maximum and minimumwindow width is in the same order as a ratio of a maximum output powerand a minimum output power, limited by boundaries of a predeterminedminimum and a predetermined maximum window width.
 12. Power controldevice for controlling an output power supplied to a discharge lampoperated by an electrical power supply, comprising: power leveldetermining means for determining an actual lamp power level; errordetermining means for determining an error between the determined lamppower level and a specified reference power level; and output powerdetermining means for maintaining an output power level supplied by theelectrical power supply to the lamp if the error falls within aspecified window and for adjusting the output power level supplied bythe electrical power supply to the lamp towards said reference powerlevel if the error falls outside the specified window; wherein theoutput power determining means comprise means for determining thereference power level on basis of a prestored nominal lamp power leveland a dimming level, which is input to the output power determiningmeans.
 13. Power control device for controlling an output power suppliedto a discharge lamp operated by an electrical power supply, comprising:power level determining means for determining an actual lamp powerlevel; error determining means for determining an error between thedetermined lamp power level and a specified reference power level; andoutput power determining means for maintaining an output power levelsupplied by the electrical power supply to the lamp if the error fallswithin a specified window and for adjusting the output power levelsupplied by the electrical power supply to the lamp towards saidreference power level if the error falls outside the specified window;wherein the output power determining means comprise means foriteratively adjusting the output power level with a first correction ifthe error is outside the window, and maintaining the output power levelif the error is inside the window; and wherein the output power meanscomprise means for adjusting the output power level supplied by thepower supply with a second correction respectively if the error isinside the main window, but outside a subwindow of the main window, thesecond correction being smaller than the first correction respectively.14. Power control device according to claim 13, wherein said first andsecond corrections are factors which are prestored in the output powermeans.
 15. Power control device for controlling an output power suppliedto a discharge lamp operated by an electrical power supply, comprising:power level determining means for determining an actual lamp powerlevel; error determining means for determining an error between thedetermined lamp power level and a specified reference power level; andoutput power determining means for maintaining an output power levelsupplied by the electrical power supply to the lamp if the error fallswithin a specified window and for adjusting the output power levelsupplied by the electrical power supply to the lamp towards saidreference power level if the error falls outside the specified window;wherein the output power determining means comprise means foriteratively adjusting the output power level with a first correction ifthe error is outside the window, and maintaining the output power levelif the error is inside the window; and wherein the output power levelsupplied to the lamp satisfies:P _(n) =P _(n-1) +K _(p)(E _(n) −E _(n-1))+K _(i) E _(n) wherein P_(n)is the (output power level supplied to the lamp on time n, P_(n-1) isthe (output power level supplied to the lamp on time n−1, E_(n) andE_(n-1) are the error on time n and n−1 respectively, K_(p) is aproportional gain and K_(i) is an integrating gain.
 16. Apparatus forsupplying power to a discharge lamp, comprising: an electrical powersupply for supplying power to the lamp; power level determining meansfor determining an actual level of the lamp power; error determiningmeans for determining an error between the determined lamp power leveland a specified reference power level; output power determining means,connected to the power supply for controlling the output power so as toadjust the output power to be supplied to the lamp towards saidreference power level only if the error falls outside a specifiedwindow, wherein said reference power level is a determined by a presetnominal lamp power and an input dimming level.
 17. Apparatus accordingto claim 16, wherein a DC power supply (UDC) is controllable and thepower level determining means control the output voltage (UDC) of the DCpower supply as to adjust the output power supplied to the lamp. 18.Apparatus according to claim 16, wherein an operation frequency (atGHB1, GHB2) is controllable and the power determining means control theoutput voltage (UDC) of the DC power supply so as to adjust the outputpower supplied to the lamp.
 19. Apparatus according to claim 16, whereinthe power supply is a switched-mode power supply (SMPS).
 20. Apparatusaccording to claim 16, wherein the power supply is of a constantfrequency pulse width modulation (PWN) type.
 21. Apparatus according toclaim 16, comprising a power control device.
 22. Method of controlling apower supplied to a discharge lamp operated by an electrical powersupply, comprising: determining an actual power level consumed by thelamp; determining an error between the actual lamp power level and aspecified reference power level; if the error falls within a specifiedwindow, maintaining the actual Output power level supplied to the lamp;if the error falls outside the specified window, adjusting the actualoutput power level supplied to the lamp towards said reference powerlevel, wherein said reference power level is a determined by a presetnominal lamp power and an input dimming level.
 23. Method according toclaim 22, wherein a window width is dependent on the specified referencepower level.
 24. Method according to claim 22, wherein a window width isdecreased towards low reference power levels and increased towards highreference power levels.
 25. Method of controlling a power supplied to adischarge lamp operated by an electrical power supply, comprising theacts of: determining an actual power level consumed by the lamp;determining an error between the actual lamp power level and a specifiedreference power level; if the error falls within a specified window,maintaining the actual output power level supplied to the lamp; if theerror falls outside the specified window, adjusting the actual outputpower level supplied to the lamp towards said reference power level;wherein a window width is variable between a maximum window width and aminimum window width, a ratio of which is approximately 1/10 or more.26. Method of controlling a power supplied to a discharge lamp operatedby an electrical power supply, comprising the acts of: determining anactual power level consumed by the lamp; determining an error betweenthe actual lamp power level and a specified reference power level; ifthe error falls within a specified window, maintaining the actual outputpower level supplied to the lamp; if the error falls outside thespecified window, adjusting the actual output power level supplied tothe lamp towards said reference power level; wherein the reference powerlevel is a determined by a preset nominal lamp power and an inputdimming level.
 27. Power control for controlling an output powersupplied to a discharge lamp operated by an electrical power supply,comprising: power level determining means for determining an actual lamppower level; error determining means for determining an error betweenthe determined lamp power level and a specified reference power level;and output power determining means for maintaining an output power levelsupplied by the electrical power supply to the lamp if the error fallswithin a specified window and for adjusting the output power levelsupplied by the electrical power supply to the lamp towards saidreference power level if the error falls outside the specified window;wherein the output power determining means comprise means foriteratively adjusting the output power level with a first correction ifthe error is outside the window, and maintaining the output power levelif the error is inside the window; and wherein a duty cycle of theoutput power level supplied to the lamp satisfies:P _(n) =P _(n-1) +K _(p)(E _(n) −E _(n-1))+K _(i) E _(n) wherein P_(n)is the duty cycle of the output power level supplied to the lamp on timen, P_(n-1) is the duty cycle of the output power level supplied to thelamp on time n−1, E_(n and E) _(n-1) are the error on time n and n−1respectively, K_(p) is a proportional gain and K_(i) is an integratinggain.